In the past, there has been proposed a lighting device which includes a control switch for supplying a constant current to an LED lighting module and supplies a dual signal defined by a low-frequency burst signal constituted by high-frequency pulses to the control switch (see document 1 [JP 2006-511078 A]).
As shown in FIG. 8, such a lighting device includes a series circuit of a diode D10 connected between opposite ends of a DC power source 100 and a control switch 101 illustrated as a MOSFET.
Further, an inductor L10 and an LED lighting module 102 are connected between opposite ends of the diode D10.
A controller 103 generates a dual PWM switching signal supplied to a control input unit of the control switch 101 via an amplifier 104. This dual PWM switching signal is substantially identical to a combination of a low-frequency pulse burst signal (i.e., a low-frequency PWM switching signal component) and a high-frequency PWM switching signal component superimposed on the low-frequency pulse burst signal.
The controller 103 includes a current mode pulse width modulator 105. The current mode pulse width modulator 105 receives an LED current reference signal from a current source 106, a detection current, and a high-frequency saw-tooth wave signal.
The current mode pulse width modulator 105 generates the high-frequency PWM switching signal component supplied to one of input parts of an AND gate 107, and the AND gate 107 receives the low-frequency PWM switching signal component at the other of the input parts. An output from the AND gate 107 is supplied to a gate of the control switch 101 through the amplifier 104.
As mentioned in the above, this lighting device can change an average current flowing through the LED lighting module 102 by means of adjusting the low frequency component of the dual PWM switching signal in order to vary intensity of light emitted from the LED lighting module 102,
The dual PWM switching signal supplied to the control input unit of the control switch 101 is a logical multiplication of the low-frequency PWM signal and the high-frequency driving signal. Therefore, when the PWM signal falls in the on period of the control switch 101, the driving signal of the control switch 101 is switched to a low level. Thus, the on period of the control switch 101 has a length varied in accordance with the change in the on-duty level (duty ratio) of the PWM signal. Such a variation of the length of the on period causes a change in a current (load current) flowing through the LED lighting module 102, that is, a light output of the LED lighting module 102. Therefore, the prior device changes the duty ratio of the PWM signal to perform the burst dimming control of the LED lighting module 102.
Further, as shown in FIG. 9, there has been proposed a lighting device 1A including a control circuit 3 constituted by a general-purpose PFC (Power Factor Correction) integrated circuit. The control circuit unit 3 is designed to control a switching element Q1 included in a lighting circuit unit 2 for supplying a current to a light source unit 10. For example, such a general-purpose PFC integrated circuit is “MC33262” (available from ON Semiconductor) and “L6562” (available from ST Microelectronics). The following explanation referring to FIG. 9 is made to the lighting device 1A.
This lighting device 1A includes mainly the lighting circuit unit 2, the control circuit unit 3, and current detection units 41 and 42. The switching regulator 2 is configured to decrease a DC voltage outputted from a DC power source E1 and supply a current I1 to the light source unit 10. The control circuit unit 3 is configured to control an output of the switching regulator 2. The current detection units 41 and 42 are configured to measure the current I1.
In the switching regulator 2, a series circuit of the light source unit 10, an inductor L1, the switching element Q1, and a resistor R1 is interposed between opposite ends of the DC power source E1.
Further, there is a diode D1 which is connected in parallel with a series circuit of the light source unit 10 and the inductor L1. The diode D1 is used for supplying energy stored in the inductor L1 (a regeneration current from the inductor L1) to the light source unit 10 in the off period Toff of the switching element Q1 constituted by an n-channel MOSFET.
The switching regulator 2 has the above configuration acting as a step-down chopper circuit. The switching regulator 2 obtains an input from the DC power source E1. The switching regulator 2 supplies the current I1 to the light source unit 10 in response to an on-off operation of the switching element Q1, thereby lighting the light source unit 10.
The light source unit 10 is constituted by plural (three in the illustrated instance) light emitting diodes 10a connected in series with each other. Besides, the number of the light emitting diodes 10a constituting the light source unit 10 is not limited to two or more. The light source unit 10 may be constituted by the single light emitting diode 10a. The light emitting diode 10a is used as a light emitting element constituting the light source unit 10. The light source unit 10 may be constituted by other kinds of light emitting elements (e.g., organic EL elements).
The current detection unit 41 is constituted by the resistor R1 connected in series with the switching element Q1. The current detection element 41 outputs a voltage across the resistor R1 to the control circuit unit 3 as a detection value (detection voltage Va) of the current I1 flowing in the on period of the switching element Q1.
Further, the current detection unit 42 is constituted by a secondary winding n2 of the inductor L1. The current detection element 42 outputs a voltage induced in the secondary winding n2 to the control circuit unit 3 as a detection value (detection voltage Vzcd) of the current I1 flowing in the on period of the switching element Q1.
The control circuit unit 3 is constituted by a driving circuit unit 31, a flip-flop 32, a comparator 33, a zero-current detection circuit 34, a starter 35, and an OR circuit 36. The control circuit unit 3 turns on and off the switching element Q1 to control the current I1 based on the detection values of the current detection units 41 and 42, thereby operating the lighting device 1 at a critical mode.
The comparator 33 has a non-inverting input terminal receiving the reference voltage Vref1, and an inverting input terminal connected to the high voltage side of the resistor R1 via the resistor R2 to receive the detection voltage Va of the current detection unit 41. Further, the comparator 33 has an output terminal connected to an R terminal of the flip-flop 32.
Additionally, when the current I1 flowing through the resistor R1 is increased and then the detection voltage Va exceeds the reference voltage Vref1 in the on period of the switching element Q1, an output signal (reset signal) of the comparator 33 is changed from a low level to a high level.
The zero-current detection circuit 34 has an input terminal connected to one end of the secondary winding n2 of the inductor L1 to receive the detection voltage Vzcd of the current detection unit 42 at the input terminal. When the current (regeneration current) I1 flowing through the inductor L1 is decreased and then the detection voltage Vzcd falls below the threshold voltage Vth in the off period of the switching element Q1, the zero-current detection circuit 34 outputs a set signal constituted by a pulse wave to the OR circuit 36.
The flip-flop 32 is an RS flip-flop, and has an S terminal connected to an output terminal of the OR circuit 36, the R terminal connected to the output of the comparator 36, and a Q terminal connected to the driving circuit unit 31. The driving circuit unit 31 generates the driving signal S1 for turning on and off the switching element Q1 based on the output signal of the flip-flop 32.
Additionally, the OR circuit 36 has one input terminal connected to the output terminal of the zero-current detection circuit 34 and the other input terminal connected to an output terminal of the starter 35.
The starter 35 monitors an output of the flip-flop 32. When the output signal of the flip-flop 32 is kept at a low level for a predetermined period, the starter 35 starts to periodically output a set signal constituted by a pulse wave to the OR circuit 36. Therefore, when the set signal is outputted from any one of the zero-current detection circuit 34 and the starter 35, the OR circuit 36 outputs a set signal to the flip-flop 32.
Upon detecting an edge of the set signal inputted into the S terminal, the flip-flop 32 is changed to a set state and the flip-flop 32 switches a signal level of the output signal to a high level. Further, when a reset signal having a high level is inputted into the R terminal, the flip-flop 32 is changed to a reset state and the flip-flop 32 keeps the output signal at the low level. While the flip-flop 32 has the reset state, the flip-flop 32 keeps the output signal at the low level irrespective of input of the set signal.
When the output signal of the flip-flop 32 has the high level, the driving circuit unit 31 changes a signal level of the driving signal S1 outputted to the switching element Q1 to a high level so as to turn on the switching element Q1. When the output signal of the flip-flop 32 has the low level, the driving circuit unit 31 changes the signal level of the driving signal S1 to a low level so as to turn off the switching element Q1.
In brief, upon judging that the current I1 is increased and the detection voltage Va of the current detection unit 41 exceeds the reference voltage Vref1 while the switching element Q1 is turned on, the control circuit unit 3 changes the state of the flip-flop 32 to the reset state, and turns off the switching element Q1.
In contrast, upon judging that the current I1 is decreased and the detection voltage Vzcd of the current detection unit 42 falls below the threshold voltage Vth while the switching element Q1 is turned off, the control circuit unit 3 changes the state of the flip-flop 32 to the set state, and turns on the switching element Q1.
The control circuit unit 3 performs such an on-off operation of the switching element Q1 to control the current I1.
Further, the control circuit unit 3 performs the on-off operation of the switching element Q1 intermittently in accordance with a dimming signal S2 outputted from a dimming signal generation unit 5, thereby performing the burst dimming control of the light source unit 10.
The dimming signal S2 is constituted by a low-frequency PWM signal defined as a binary signal having a high level (first state) and a low level (second state).
The control circuit unit 3 performs the on-off operation of the switching element Q1 when the dimming signal S2 has the high level, and does not perform the on-off operation of the switching element Q1 when the dimming signal has the low level.
To perform the aforementioned dimming control, the lighting device 1A includes a dimming control unit 6.
The dimming control unit 6 is constituted by a resistor R3, a switching element Q2, and a control power source E2. The control power source E2, the switching element Q2, and the resistors R1 to R3 constitute a series circuit.
Further, the switching element Q2 is turned on and off in accordance with the signal level of the dimming signal S2 for superimposing a predetermined voltage on the detection voltage Va. In other words, the detection voltage Va is increased by the predetermined voltage.
Interposed between the switching element Q2 and the dimming signal generation unit 5 is an inverting element 51. Thus, a signal (hereinafter referred to as “dimming signal S2a”) obtained by inverting the dimming signal S2 is inputted into the switching element Q2.
When the dimming signal S2a has the high level (the dimming signal S2 has the low level), the switching element Q2 is turned on. When the dimming signal S2a has the low level (the dimming signal S2 has the high level), the switching element Q2 is turned off.
The control power source E2 is configured to outputs a control voltage VDD. When the switching element Q2 is turned on, a current flows from the control power source E2 to the resistors R1 to R3 via the switching element Q2. As a result, the predetermined voltage is superimposed on (added to) the detection voltage Va applied to the inverting input terminal of the comparator 33. It is assumed that the resistors R2 and R3 have resistances r2 and r3, respectively. The resistances r2 and r3 are selected to satisfy a relation of r2/(r2+r3)>Vref1/VDD while the switching element Q2 is turned on. Thus, the increased detection voltage Va (the sum of the original detection voltage Va and the predetermined voltage) exceeds the reference voltage Vref1.
Consequently, the reset signal outputted from the comparator 33 has the high level, and the flip-flop 32 keeps having the reset state. In brief, when the switching element Q2 is turned on, the switching element Q1 is kept turned off and the light source unit 10 is switched to an extinction state.
Additionally, when the switching element Q2 is turned off, a path of an output current of the control power source E2 is broken. The voltage is not superimposed on the detection voltage Va. As a result, the control circuit unit 3 performs the aforementioned on-off operation of the switching element Q1. In brief, when the switching element Q2 is turned off, the on-off operation of the switching element Q1 is executed and the light source unit 10 is switched to a lighting state.
As described in the above, the intermittent control of the on-off operation of the switching element Q1 is performed in accordance with the on-duty level (duty ratio) of the dimming signal S2. Therefore, the burst dimming control of dimming the light source unit 10 can be implemented.
The following explanation referring to FIG. 10 (a) to (d) is made to a sequence of operations of the lighting device 1A.
When the sequence proceeds to an on period Ton in which the dimming signal S2 has the high level, the set signal for activation is inputted into the OR circuit 36 from the starter 35, and the other set signal is inputted into the S terminal of the flip-flop 32 from the OR circuit 36. As a result, the flip-flop 32 is switched to the set state, and the output signal from the flip-flop 32 is changed to the high level. Consequently, the driving signal S1 of the driving circuit unit 31 is switched to the high level, and the switching element Q1 is switched from the off state to the on state. Thus, a current flows from the DC power source E1 through the light source unit 10, the inductor L1, the switching element Q1, and the resistor R1 to the DC power source E1 in this order. Consequently, the current I1 is increased (see FIG. 10 (d)).
The increase in the current I1 causes an increase in the voltage across the resistor R1, that is, the detection voltage Va of the current detection unit 41 (see FIG. 10 (c)). In this situation, since the switching element Q2 has the off state, no voltage is superimposed on (added to) the detection voltage Va.
Subsequently, when the detection voltage Va reaches the reference voltage Vref1, the output of the comparator 33 is inverted, and then the reset signal having the high level is inputted into the R terminal of the flip-flop 32. Consequently, the flip-flop 32 is switched to the reset state, and the output signal is switched from the high level to the low level. Further, the driving signal S1 of the driving circuit unit 31 is also switched from the high level to the love level, and then the switching element Q1 is switched from the on state to the off state (see FIG. 10 (c)).
When the switching element Q1 is switched to the off state, energy stored in the inductor L1 causes a regeneration current flowing through a closed path of the diode D1, the light source unit 10, and the inductor L1. Specifically, such a regeneration current is outputted from the inductor L1 and passes through the diode D1 and thereafter the light source unit 10 and returns to the inductor L1.
The current I1, that is, the current flowing through the inductor L1 is gradually decreased and finally becomes zero (see FIG. 10 (d)). Besides, a broken line in FIG. 10 (d) shows a peak value Ith of the current I1.
When the current flowing through the inductor L1 reaches zero, the inductor L1 causes a reverse current, and then electric charges stored in the switching element Q1 is discharged via a parasitic capacitance of a device (e.g., the diode D1). As a result, a drain-source voltage of the switching element Q1 is decreased. Consequently, a reverse of a voltage applied across the inductor L1 occurs. The zero-current detection circuit 34 detects the reverse of the voltage on the basis of a voltage induced in the secondary winding n2.
Upon detecting the reverse of the voltage of the inductor L1 (an event where the detection voltage Vzcd falls below the threshold voltage Vth), that is, a zero crossing of the current flowing through the inductor L1, the zero-current detection circuit 34 outputs the set signal to the OR circuit 36.
Thus, the OR circuit 36 outputs the set signal to the S terminal of the flip-flop 32. The flip-flop 32 is switched to the set state and the output signal from the flip-flop 32 is switched from the low level to the high level. Further, the driving signal S1 of the driving circuit unit 31 is also switched from the low level to the high level, and then the switching element Q1 is changed from the off state to the on state (see FIG. 10 (c)).
With performing the on-off operation of the switching element Q1 defined as a repetition of a series of operations (turning on and off of the switching element Q1), the control circuit unit 3 operates the switching element Q1 at a critical mode. Each lighting diode 10a of the light source unit 10 emits light while the current I1 flows through the light source unit 10.
Thereafter, when the sequence proceeds to the on period Toff in which the dimming signal S2 has the low level, the switching element Q2 is switched from the off state to the on state, and the predetermined voltage is superimposed on the detection voltage Va. As a result, the resultant (increased) detection voltage Va exceeds the reference voltage Vref1. Consequently, the reset signal which is inputted into the R terminal of the flip-flop 32 is kept at the high level, and the flip-flop 32 is kept in the reset state. Thus, the output signal from the flip-flop 32 is switched to the low level. Therefore, the driving signal S1 of the driving circuit unit 31 is also switched to the low level, and the switching element Q1 is kept turned on.
After the signal level of the dimming signal S2 is inverted again and the sequence proceeds to the on period Ton, the current I1 does not flow through until the starter 35 outputs the set signal. Thus, each light emitting diode 10a of the light source unit 10 is turned off.
To adjust the luminance of the light source unit 10, the intermittent control of the on-off operation of the switching element Q1 which repeats the aforementioned sequence of the operations based on the dimming signal S2 defined as the low-frequency PWM signal, that is, the burst dimming control, is performed. Therefore, with changing the on-duty level (duty ratio) of the dimming signal S2, it is possible to change the proportion of lighting time and extinction time to whole time. Thus, the dimming control of the light source unit 10 can be achieved.
Note that the general-purpose integrated circuit (IC) used for constituting the control circuit unit 3 includes the starter 35. The starter 35 is configured to output the set signal after a lapse of a predetermined period (hereinafter referred to as “starting period Tstr”) from the time at which the on-off operation is terminated in the off period Toff. Therefore, when the aforementioned burst dimming control is performed by use of such a general-purpose integrated circuit, and when the off period Toff is selected to be shorter than the starting period Tstr, the duty ratio unavailable for the dimming control is likely to exist.
FIG. 11 (a) to (c) shows an instance where the off period Toff in which the on-off operation of the switching element Q1 is terminated is longer than the starting period Tstr of the starter 35. In this instance, the starter 35 is activated in the off period Toff and outputs the set signal periodically. Therefore, when the sequence proceeds to the on period Ton, the reset state of the flip-flop 32 is canceled, and the starter 35 outputs the set signal. Consequently, the on-off operation of the switching element Q1 is restarted immediately.
FIG. 12 (a) to (c) shows an instance where the off period Toff is shorter than the starting period Tstr. In this instance, even when the sequence proceeds from the off period Toff to the period Ton, the starter 35 does not output the set signal until the starting period Tstr elapses. After a lapse of the starting period Tstr, the starter 35 outputs the set signal and then the on-off operation of the switching element Q1 is restarted.
In brief, when the off period Toff is shorter than the starting period Tstr, the following problem will occur. That is, it is impossible to restart the on-off operation until the starting period Tstr elapses. The starting period Tstr depends on the general-purpose IC used for constructing the control circuit unit 3. For example, the L6562A available from ST Microelectronics has the starting period Tstr of typically 190 μs.
To perform the burst dimming control of the light source unit 10 at the relatively high dimming level, it is necessary to select the relatively high on-duty level (duty ratio). However, in a range of the duty ratio in which the off period Toff is shorter than the starting period Tstr, the dimming level is not changed. Besides, when the on duty level has 100%, the dimming signal S2 always has the high level and the starter 35 does not operate. Therefore, the aforementioned problem does not occur.
For example, the control circuit unit 3 is constituted by use of the L6562A available from ST Microelectronics having the starting period Tstr of 190 μs, and the dimming signal S2 has a frequency of 1 kHz. In this instance, in a range in which the on-duty level of the dimming signal S2 is greater than about 80% and is less than 100%, the dimming level of the light source unit 10 is not changed.
For example, to avoid the above problem, parameters of the lighting device can be selected such that the light output corresponding to the dimming signal S2 having the on-duty level not greater than 80% is increased up to the light output of 100% without changing the on-duty level. However, this solution causes an increase in the peak current. Therefore, there will occur another problem that an energy loss is increased.